Hello vmandem,
The simplest way is to insert a blank column, then use a formula:
=A1 & ";"
You can copy this formula down to the end of your data by clicking the little square at the bottom right of the selection marquee.
The final step would be to copy the cells with the formula, then Edit...Paste Special...Values on top of the original values. You may then delete the column with the formula.
Regards,
Brad